The present invention relates to a data transmission control system and, more particularly, to a data transmission control system which allows remote transceiver stations to exchange images, digital data and like signals over an analog transmission line using modems.
In data transmission using an analog transmission line such as a telephone line, it is a common practice to execute a training sequence for a modem when connection has been set up between remote transceiver stations by a call. As soon as various parameters are converged by the training, a test signal transmission sequence is performed before the transmission of information in order to predetermine a data transmission rate. For example, in accordance with the facsimile transmission control procedure presented by the Consultive Committee of International Telegraph and Telephone (CCITT), Recommendation T.30, training for a modem of the group 3 (G3) is confirmed and, then, a training check field (TCF) is defined as a test signal for setting up a data transmission rate. The TCF consists of a signal in which ZEROs sequentially appear for the duration of 1.5 seconds. The receiving station, receiving the TCF and when found many transmission errors, requests the transmitting station to shift down the data transmission rate.
As stated in the Recommendation T.30, if a modem at the receiving station fails to adapt itself to test data which is delivered thereto from the transmitting station for 1.5 seconds at the transmission rate of 9,600 bits per second (bps), for example, the receiving station notifies it to the transmitting station while the transmitting station responds to the notification by shifting down the transmission rate to 7,200 bps and then repeating the same test sequence. If the modem at the receiving station is failed to such a degree as to require further shift-down from 4,800 bps to 2,400 bps, 6 seconds will be consumed in total for the delivery of test data alone. The resulting increase in the total protocol time is undesirable because it lowers the data transmission efficiency of the whole system.
Meanwhile, a communication control system is known in which test data is sequentially delivered from a transmitting station to a receiving station at all the transmission rates which a modem can accommodate, the delivery being repeated until the receiving station completes equalization (Japanese Patent Laid-Open Publication Nos. 52-94715/1977 and 52-94716/1977). Such a system is not fully acceptable, however, considering a possible occurrence that, even after equalization has been completed at a given transmission rate, test data of lower transmission rates are sent out, increasing the total protocol time.